The invention relates to a hand held powered reciprocating saw, and in particular to a mechanism for holding an object while it is cut by the blade of the reciprocating saw.
A typical reciprocating saw is disclosed in GB 2 234 033. The reciprocating saw disclosed therein comprises a housing having a handle portion and a front portion from which a saw blade extends. The saw blade is guided for reciprocating movement and is connected to a drive mechanism within the housing which drives the saw blade to reciprocate. A typical example of saws of this kind are pruning saws used for cutting the branches of trees, bushes and the like. In this field of application the user holds the saw at the handle portion and brings the reciprocating saw blade into contact with the branch to be cut. The reciprocating saw blade is slowly moved through the branch while cutting it. The reciprocating movements of the saw blade exert forces on the object which is sawn, i.e. the object, for example a branch, tries to follow the reciprocating movement of the saw blade since there is a considerable friction between the reciprocating saw blade and the branch. Therefore, the user has to grip the branch and hold it tightly with one hand against the saw, while the pruning saw is held in the other hand in order to achieve an accurate cut. The need to hold the object, e.g. a branch, to be sawn with the reciprocating saw can make the saw very inconvenient to use.
Another problem or inconvenience arises due to the fact that the user has to absorb the oscillating forces which act between the reciprocating saw and the object which is sawn. When the user grips the branch to be cut and holds the reciprocating saw with the other hand he has to absorb the reciprocating forces acting between the branch and the saw with his arms. On the one hand this makes use of the saw tiring, and on the other hand, since the user cannot hold the branch completely fixed, the cut can be inaccurate if the saw is not steadily held in the same position with respect to the branch.
The problems outlined above do not apply to pruning saws only but to reciprocating saws in general. Whenever the object to be sawn has a certain flexibility or is not fixed and so has to be held by the user, for example when a rod has to be cut into pieces of shorter length, problems as described above arise.
The problems are further exacerbated when the branches of trees and bushes high above a user are cut using a reciprocating saw mounted on an extension arm or pole. In this case a user cannot reach the branch to be cut to hold it while it is cut. Thus, the reciprocating movement of the blade is transferred to the branch and the branch reciprocates also. This reduces the cutting performance of the saw, can lead to inaccurate cuts being made and tends to increase the length of time required to complete the task in hand.
It is therefore desirable to have a clamping device to hold the object to be saw stationary to prevent it from following the reciprocating movement of the saw blade whilst allowing the saw blade to cut through the object.
One such clamping device for a compass saw is disclosed in GB 2,182,282 and comprises a clamping holder in the form of a part circular bracket which is pivotally mounted at its centre on the body of the saw. On one arm of the part circular bracket is a clamping jaw having two inwardly facing clamping surfaces. On the other arm of the part circular bracket is a clamping part which comprises a threaded shaft which screws through a corresponding threaded hole, the direction of travel of the shaft as it screws into or out of the hole being within the plane of the clamping device either towards or away from the clamping jaw. In operation, the workpiece to be cut is inserted into the region bounded by the part circular bracket and is clamped between the clamping part and the clamping jaw by screwing the clamping part towards the clamping jaw and trapping the workpiece in between. As the blade of the saw cuts through the workpiece, the clamping device pivots to allow the workpiece to remain in contact with the cutting edge of the saw blade.
The clamping device disclosed in GB 2,182,282 must be firmly attached to the object to be sawn by the user prior to commencing the sawing operation. This can be difficult if the object to be sawn is located in an awkward position, for example, if it is a branch high up in a tree. The user is required to use both hands whilst attaching the clamp, one to hold the clamping device against the object to be cut whilst the other tightens the clamping part to hold the object to be cut. Furthermore, as the clamping device is firmly attached to the object to be cut, the only movement relative to the object to be cut which the body of the saw can make whilst cutting through the object to be cut is a pivotal movement. This may be undesirable in certain situations where there is insufficient room for the body of the saw to pivot.
DE 4429408 also disclosed a clamping device for a compass saw. The clamping device comprises a structure similar to a pipe wrench which is pivotally mounted on the saw housing. The object to be sawn is clamped by turning a knob which in turn rotates a threaded spindle to bring two clamping surfaces of the wrench into engagement with and thus hold the object. The clamping device disclosed in DE 4429408 suffers from the same problems as those suffered by the clamping device in GB 2,182,282.
It is an object of the present invention to overcome the problems outlined above. In particular it is an object to provide a hand held reciprocating saw which allows the user to hold the saw with one hand without the need to hold the object to be cut with the other hand in order to prevent an oscillating movement of that object while it is cut.
According to a first aspect of the present invention there is provided a hand-held powered saw which saw has a reciprocating cutting blade and comprises:
a clamping arm which is rotatably mounted on the saw by means of a one way rotary clutch such that the arm can freely rotate in one direction only; and
a support member which co-operates with the clamping arm to hold an object to be cut;
wherein the centre of mass of the clamping arm is spaced apart from the axis of rotation of said rotary one way clutch so that the vibration of the saw when the saw is in use induces a progressive rotation of the clamping arm.
The vibration of the saw induces a stepwise progressive rotation of the clamping arm due to its moment of inertia. Since the clamping arm can rotate in one direction only, movements caused by the moment of inertia while the saw as a whole is vibrating results in stepwise rotational movements in one direction only. Thus, when the saw is activated and held by the user with the saw blade reciprocating, the clamping arm performs a progressive rotational movement. If the user holds the saw close to a branch so that the branch contacts the support member the rotational movement of the clamping arm has the effect that the clamping arm closes and clamps the branch resting on the support member. If the object to be cut is not properly clamped when it comes into contact with the saw blade, the initial movement of the object as it follows the reciprocating movement of the blade causes greater vibrations which quickly accelerates the clamp into place. Thus, the user only has to position the saw such that the branch can be held between support member and clamping arm, and the clamping mechanism will close automatically driven by the reciprocating movement of the saw. The clamping arm and support member can be arranged so that they co-operate with each other to firmly clamp the object to be cut between the two. However, the clamping arm and support can alternatively be arranged so that they co-operate with each other to hold the object to be cut against the cutting edge of the saw blade. In either arrangement the saw blade can then be moved relative to the object, by a variety of means, in the direction of cut so that the reciprocating saw blade can move through the object held between the support member and clamping arm to cut the object. Objects such as branches can be sawn while holding the reciprocating saw with one hand without having to hold the branch as well.
The forces in the direction of blade reciprocation between the object and the saw which are caused by the reciprocating movement of the saw blade through the object can be absorbed by the holding of the object between the clamping arm and support member. Thus, the reciprocating saw according to the invention is easier and more convenient to use than prior art saws.
As the blade cuts through the object to be cut, the clamping arm may rotate in order to maintain hold with the co-operation of the support member, on the object to be cut and thus ensure that any reciprocating movement of the object to be cut is prevented or substantially reduced whilst the object is being cut.
A preferred means of moving the saw blade relative to an object held between the clamping arm and support member uses a slide element which is slideably mounted on the saw for sliding movement in a direction across the blade wherein the clamping arm is mounted on the slide element so that the sliding movement of the slide element and clamping arm guides an object held between the support member and the clamping arm across the cutting blade. Preferably, the support member is also mounted on the slide element so that the object held between the support member and the clamping arm does not have to move across the support member as it is guided across the cutting blade.
The support member may comprise the reciprocating saw blade. The shape of the clamping arm can be designed so that it grips and holds the object to be cut against the cutting edge of the blade without the assistance of any other part of the saw. The clamping arm would prevent or substantially reduce the reciprocating movement of the object to be cut caused by the reciprocating movement of the saw blade. As the blade cut through the object to be cut, the clamping arm would rotate ensuring that the object to be cut is securely held by the clamping arm to prevent or substantially reduce the reciprocating movement of the object. By using the saw blade as the support, it is possible to design a reciprocating saw with a clamping mechanism so that the object to be cut can be held against any part of the length of the reciprocating blade.
Where the clamping arm co-operates with the support member to hold the object to be cut against the cutting edge of the saw blade, ideally, the shape of curvature of the surface of the clamping arm which engages an object to be cut (hereinafter referred to as the holding surface) and location of the axis of rotation of the clamping arm are such that when an object of circular cross section is first cut by the reciprocating blade, the tangent to the surface of the object at the point on the surface where the clamping arm first makes contact with the object is at an angle of between 30xc2x0 and 60xc2x0 and preferably between 40xc2x0 and 50xc2x0 to a plane which passes through the length of the blade which plane is 90xc2x0 to the plane of the blade. This has the benefit of providing a clamping arm which can securely hold objects to be cut against the cutting edge of the blade for a variety of such objects which vary greatly in their cross sectional areas whilst preventing or severely reducing the reciprocal movement of the object caused by the reciprocal movement of the blade.
In a preferred embodiment of the present invention, the clamping arm is mounted on the saw via a reverse slip clutch which allows rotation of the clamping arm in the reverse direction to the direction of free rotation of the rotary one way clutch if a reverse torque is exerted exceeding a specified threshold. This embodiment allows the user to remove the object from the clamping mechanism in a simple manner by manually turning the clamping arm in a direction opposite to the direction of rotation of the rotary one way clutch. Ideally, the one way rotary clutch and the reverse slip clutch are co-axially mounted to simplify the construction.
Furthermore, it is preferable that the axis of rotation of the rotary one way clutch is oriented essentially perpendicular to the plane of the blade.
Ideally, the clamping arm is made from a plastics material having a metal weight located towards the end of the clamping arm distant from the rotary one way clutch. By making the clamping arm from plastic, it makes the manufacture of the clamping arm cheaper and easier. Furthermore, it reduces the weight of the clamping arm which in turn reduces the overall weight of the saw making it easier for the user to use. By adding a metal weight towards the end of the clamping arm away from the one way rotary clutch, the centre of mass of the clamping arm is moved a greater distance from the axis of rotation of the clamping arm. This results in a greater moment of inertia of the clamping arm about the axis of rotation and therefore increases its rate of rotation.
In the plane of the clamping arm, the direction of curvature of the end of the clamping arm distant from the one way rotary clutch should preferably be opposite to that of the holding surface of the clamping arm, for example the clamping arm may be xe2x80x98Sxe2x80x99 shaped. Changing the direction of curvature of the tip region of the clamping arm prevents the clamping arm from snagging on larger branches and makes it easier to pull the saw away from a branch whilst it is still being held by the clamping mechanism.
According to the second aspect of the present invention there is provided a clamping mechanism which is mountable on a hand-held powered reciprocating saw which saw has a reciprocating cutting blade, said clamping mechanism comprising:
a clamping arm which is rotatably mounted on the clamping mechanism by means of a one way rotary clutch such that the arm can freely rotate in one direction only;
wherein the centre of mass of the clamping arm is spaced apart from the axis of rotation of said rotary one way clutch.